1. Field of the Invention
The present invention relates two non-aqueous methods of making metal-terephthalate (metal-TPA) polymeric crystals. A first embodiment involves a chemical reaction between terephthalic acid (TPA) and a metal oxide, such as zinc oxide or magnesium oxide. A second embodiment involves a compound such as magnesium hydroxide or calcium hydroxide that decomposes in the general range of the melting point of polyester ethylene terephthalate (PET).
2. Description of the Art
U.S. Pat. No. 5,164,458 discloses a process for producing calcium terephthalate elongated fibers having an average particle size of about 100 mesh synthesized by an aqueous process.
U.S. Pat. No. 5,380,593 provides 9 examples of multistep aqueous based syntheses, for making calcium as well as zinc and barium terephthalate fibers. It adds the distinction that these are anhydrous fibers or rods and not hydrated plates as produced by simpler aqueous synthesis processes. The distinction is important because the hydrated plates do not function in most polymer systems because the release of water causes problems. Various sizes of the fibers are given, the smallest being 75 microns in length.
It has been known to convert used passenger vehicle tires into useful byproducts through a pyrolysis process. See generally U.S. Pat. Nos. 6,835,861 and 6,833,485. These patents disclose a process of heating the material to a temperature where the original molecules are rearranged into more stable thermal species.
U.S. Pat. No. 5,446,112 discloses a multistep synthesis strategy for making metal terephthalates resins. It begins with an aromatic monocarboxylic acid which reacts with a metal oxide. Water is then distilled off followed by a reaction with a dialkyl ester of an aromatic acid. This yields a metal-terephthalate resin and a byproduct alkyl ester. No conditions are given for the final step. The byproduct alkyl ester is distilled away in this strategy. The only information on the form and purity of the resin product is that it contains less than 10 ppm of metal halide.
U.S. Pat. No. 5,254,666 is directed toward a reaction of polyester ethylene terephthalate with metal compounds to recover terephthalic acid, as well as the polyol of the polyester. The upper limit of temperatures employed is 180° C. The temperature cited is well below the melting point of polyester ethylene terephthalate. The examples mention potassium and sodium, which are monovalent and belong to the class known as alkali metals. The patent states that the product powder is dissolved in water. The patent discloses alkaline earth metals, which are polyvalent and would include calcium and magnesium, but the fact that the product dissolved in water, shows that the product could not have been a polyvalent metal polymer as disclosed in the present invention.
U.S. Pat. No. 5,545,746 is also directed toward recovery of terephthalic acid, as well as the polyol of the polyester. The examples only mention potassium and sodium. The patent discloses dissolving the product in water which again shows that the product could not have been a polyvalent metal polymer as discussed in the present invention.
U.S. Pat. No. 7,825,213 discloses making metal-terephthalate polymer by reacting PET with at least one material selected from a group of polyvalent metal compounds in a first reaction zone, which provides a non-aqueous PET melt environment at temperatures of about 270° C. and 380° C. Subsequently, the reaction product from the first zone in the reactor is introduced into a second zone for processing at about 400° C. to 600° C. In this second zone unreacted PET and undesired byproducts are destroyed and leave the zone as vapor. The yields of metal-terephthalates reported by U.S. Pat. No. 7,825,213 are about 10% to 40%. The disclosure of U.S. Pat. No. 7,825,213 is expressly incorporated herein by reference.
A paper by P. Baker and R. F. Grossman, “Properties and Reactions of Metal Terephthalates,” Journal of Vinyl Technology, Volume 11, No. 2, pp. 59-61, June, 1989 discloses the unique properties of polyvalent metal terephthalates. One unique property is their high thermal stability which results in high decomposition temperatures. Typically polyvalent metal terephthalates do not decompose until about 600° C. The paper explains this unusual stability by showing the interlinked structure of the longchain polyvalent metal terephthalate molecules. The paper also makes a sharp distinction between monovalent terephthalate salts and divalent terephthalate metals. Monovalent terephthalate salts are water soluble. Multivalent metals make a terephthalate polymer which exhibits only trace solubility. Both the high thermal stability and the insolubility in water follow from the interlinked structure.
A number of patents disclose uses of metal terephthalates. U.S. Pat. No. 4,952,634 discloses the use of polymeric polyvalent metal polycarboxylic acids such as zinc terephthalate providing a crosslinking reaction with carboxylated rubber. It also discloses decomposition temperatures for many of these metal-TPA polymers. See also U.S. Pat. Nos. 4,983,688; 5,164,458; 5,380,593; 5,446,112; and 5,475,045. All of these patents employ aqueous processing which is a fundamentally different way of producing metal-TPA than U.S. Pat. No. 7,825,213. Other patents such as U.S. Pat. No. 5,254,666 and U.S. Pat. No. 5,545,746 are directed toward recovery of terephthalic acid and polyols. They differ from U.S. Pat. No. 7,825,213 by operating at lower temperatures and use of monovalent metals such as potassium and sodium.
A paper by B. V. L'vov, A. V, Novichikhin, and A. O. Dyakov, “Mechanism of Thermal Decomposition of Magnesium Hydroxide” Thermochimica Acta, 1998, 315, 135-143 discusses the decomposition of magnesium hydroxide, and how it produces high energy vapor molecules, not simply MgO solid molecules as generally reported in the literature. This is important because the MgO vapor is much more reactive. However the paper provides no suggestion that these high energy molecules could be utilized in chemical reactions, such as those with PET or TPA.
The book by B. V. L'vov, “Thermal Decomposition of Solids and Melts”, Springer (2007), tabulates the decomposition of solids more generally and gives data suggesting a decomposition temperature of approximately 262° C. for magnesium hydroxide and 297° C. for calcium hydroxide. The corresponding number for barium hydroxide is 337° C. and for strontium hydroxide is 317° C. This work suggests that a number of metal hydroxides will behave similarly to the reactions described in U.S. Pat. No. 7,825,213. However, the book itself never suggests that the decomposition could be combined with the melt temperature of PET in a way that yields metal-terephthalates. An indirect use of the decomposition data in the book is that it shows that an alternate method of synthesis will be needed to make metal-terephthalic compounds from most metals as they do not have compounds that decompose in the temperature range of the PET melting point.
In spite of the foregoing known methods, compositions and understanding shown in the art, there remains a very real and substantial need for a method of making anhydrous, metal terephthalate polymeric crystals of other metals which do not offer the synergy of decomposition temperatures with PET melt temperature. There is also a need for a method that gives higher yields. This is especially true in order to efficiently utilize the terephthalic acid content in recycled polyester ethylene terephthalate plastic.